Method of prestressing reinforced concrete pipes



J 1952 F. L. FITZPATRICK 2, ,7

METHOD OF PRESTRESSING REINFORCED CONCRETE PIPES Filed March 16, 1949 ATTORNEYS Patented Jan. 15, 1952 METHOD or rncsfrnessmo esmroncan CONCRETE PIPES Frank Lionel Fitzpatrick, East Malvern, Victoria...

Australia; assignor to Rocla Limited, Sp ing: vale, Victoria, Australia, acompany of Victoria,

"Aus a a.

Application March-16, mas mar No; 81,761 r I InAustralia May 3;-'19 l8 I My invention relates to themanufacture .0 pre-stressed reinforced concrete pipes 26 cla (01. 25-7154 According to the general method ;of manufacturing reinforced concrete pipes. the reinforcin steel is held in tension while the concrete sets and hardens, so producing a state of pre-compression in the concrete of the-pipe wall. method is known to be very eiiicient.

One of the specific methods of carrying out this general procedure involves the application, to the inner surface-of a freshly made concrete pipe, of pressure by means of an expanding core. This core pressure producesthe desired state of reinforcement tension. During the application of thispressure the pipe. is held within an outer mould.

Problems arising during the application of this method include: I r

(a) The fact that the outer mwld absorbs some of the tension which it is desired to place upon the pipe reinforcement and it is diiiicult to assess the amount so absorbed;

(b) Especially in the caseof-large diameter pipes, forces of great magnitude operate on the outer mould, which for this-reason has to be made extraordinarily strong, .-making it costly and cumbersome.

The objects of my invention are, first, -to p'r'ovide an economical and effective solution to both of the problems above-stated, and, second, to ensure that the whole, or virtually the whole, of the expansive force exercised by the expanding core is absorbed in the reinforcing steel, whilst at the same time, permitting the outer mould to be of ordinary standard construction, with no abnormal strength.

Fig. 1 is a cross-sectional view of an outer mould containing a concrete pipe and a core;

Fig. 2 is an elevational view of a single twisted wire, and

Fig. 3 is an elevational view of two twisted wires.

The outer layer I of the pipe wall is first constructed, with the necessary steel reinforcement 2 embedded therein, and when this outer layer has chemically hardened adequately an inner layer 3 is applied to the wall to build up the wall to the desired thickness, after which the outer mould 4 is removed, or relaxed from close contact with the concrete, and pressure is applied outwardly, as indicated by the arrows, by an ex-- pansible core 5 to the inner surface of the pipe before the concrete of said inner layer has hardened chemically to an appreciable degree (1. e. while the concrete of said inner layer is still in such a. condition that the core pressure will not cause it to crack) this pressure being maintained until-the concrete .of said inner layer has hardenedsuflic-iently to withstand the inward pressure exerted by the tension of the reinforcement, and the core pressure is then released. The phrase this outer. layer has chemically hardened adeguately which is used above, means that the ut r ay r hasv hardened Sufficiently to prev nt mov mentof. th concrete of that layer n m tion to the reinforcement, or collapse'of that con-- crete. whensubie d tot e core pressure, The outerlayer o c et in accordan w h my invention, mu t b har ened athe stag here it can, while" unsupported by a mould,'sustain as a unit, without collapsing, the pressuretrans mitted to it by the core pressure. lSlormally this hardeni g would be r ch d, ith ea c r g. .12 13 ,241 hours after being made. The inner layer, inv accordance withmy nvention, i not applied to. theinside "of the outer layer to buildup thewall, and outward pressure is not. applied through the inner layer, until theouter layer has hardened adequately as above explained. If

- the concrete cf the outer layer had merely reached the state, where it had been pressed to a firm initial condition (i. e. without chemical hardening), it wouldbe quite incapable of -withstanding the outward pressure without slippage of the concrete of the outer layer past the reinforcement, or total collapse of the pipe, occurring because an outer mould is not used to retain said concrete in compact form during this application of outward pressure.

The outer layer, for example, in a pipe of which the wall will finally be 3 in thickness may be approximately 1 in thickness and the inner layer of concrete may be applied thereto by the centrifugal process or otherwise. Normally, adequate bonding between the two layers is attained without taking special steps for that purpose; but, if desired, the inner face of the outer layer can be roughened and coated with cement slurry prior to placing the inner layer of concrete in position. The hardening of the concrete of the inner layer is usually accelerated by heatmg.

When the core pressure is released the reinforcing steel of the outer layer, being in tension, then endeavours elastically to contract to its normal length but this is resisted by the inner layer of hardened compressed concrete, so that the latter is thrown into the desired state of precompression.

When the wire reinforcement contained in the any substantial slippage along the reinforcement.

and thus ensures that any cracks which .are formed will not be of substantial width. Cracking in concrete, when the width of the crack is restricted to the order of .005 inch or less is not considered to prejudice the protection of steel reinforcement against rusting; in ordinary conditions. For example, pro-stressed concrete pipes of said inner layer has hardened chemically to an appreciable degree, maintaining this pressure until the concrete of said inner layer has hardened sufficiently to withstand the inward pressure exerted by the tension on the steel reinforcement, and then relaxing this core pressure.

' 2. A method of pre-stressing reinforced con- .crete pipes as claimed in claim 1, in which the pipe .so formed is given a protective outer coatmg or impregnation.

"3. A method of pre-stressing reinforced concrete pipes asclaimed in claim 1, in which the made by the old wire banding process, with an exterior mortar coat applied after tensioning,

tend to fine cracking in the mortar coat when the pipes are under pressure, but this-has not prevented their use in major pipelines, especially in the United States. If desired a protective outer coating or impregnation may be applied.

Another method of -minimizing outer cracking is to make the outer layer of concretecapable of stretching, without cracking, to a greater extent than is the case with normal concrete. For example the inclusion in such concrete of 2-3% of finely divided short fibred asbestos tends to improve the stretchability of the concrete. Similarly concrete made with well distributed fine cells in the aggregate and/or in the mortar (e. g. scoriaceous aggregate and air-entrained mortar) elongates without cracking to a greater extent than ordinary concrete, s'o reducing the size of cracks I I claim:

1. A method of pre-stressing reinforced concrete pipes comprising first constructing an outer layer of the pipe wall, with the necessary steel reinforcement embedded therein, and, whenthe concrete of this outer layer has set and hardened adequately by the chemical reactions of the cement in said concrete, applying an inner layer of concrete to build up the wall of the pipe to the required thickness, removing'or relaxing the outer mould, applying pressure of sufiicient de gree to expand the outer layer and thus stress the steel reinforcement by an expansible core to the inner surface of the pipe before the concrete outer layer is formed of a concrete capable of expansion,without-cracking, to a greater extent than is the casewith ordinary concrete.

\ 4. A method of pre-stressing reinforced concrete pipes as claimed in claim 1, in which the outer-face of the concrete has fine cracks and in which the size of the finecracks in the outer face of the concrete is kept to a minimum'j by the use of twisted wire reinforcement or other form of mechanical bond to restrain the concrete against any substantial slippage along the reinforcement.

5; A method of pre-stressing reinforced concrete pipes as claimed in claim 1, in which the thickness of the outer layer is approximately one-third of the total thickness of the pipe.

6. A method of pre-stressing reinforced concrete pipes as claimed in claim 1, in which the outer layer is formed of concrete containing 23% of finely divided 'short fibred asbestos.

FRANK LIONEL FITZPATRICK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 994,496 vBerrigan June 6, 1911 2,048,253v Freyssinet July 21, 1936 2,395,216 Fitzpatrick Feb. 19, 1946 FOREIGN PATENTS Number Country Date 338,934 Great Britain Nov. 25, 1930 

